For the first time, American eels (Anguilla rostrata, shown) have been tracked on the way to their spawning grounds at sea, a migration of at least 1600 kilometers from the freshwater haunts where they matured. Neither researchers nor fishermen have ever caught an adult eel in the open ocean, but it’s clear they must spend time there because scientists discovered their presumed spawning grounds in the North Atlantic’s Sargasso Sea more than a century ago. Now, using tracking devices, researchers have finally mapped the migration routes of a few of those wriggly fish—and learned a little about their habits at sea to boot. Some of the devices measured water temperature and depth, and others measured temperature and the strength and intensity of Earth’s magnetic field. Of the 38 eels released off the southern coast of Nova Scotia, Canada, in the summers of 2012 through 2014, trackers on 28 of them eventually popped to the surface to broadcast data to the researchers via satellite (including two attached to eels that were apparently consumed by predators). Six of the animals were tracked for more than a month, with the longest migration stretching almost 1600 kilometers to a point just shy of the northern edge of the Sargasso Sea, the researchers report online today in Nature Communications. The eels apparently migrate from fresh waters on the North American continent in two phases: First, in shallow waters along the continental shelf, the wrigglers swim between surface waters and the bottom, possibly sampling the salinity and temperature of deeper waters to ascertain their location and route. Then, after the eels pass the edge of the shelf, they make a beeline southward, swimming near the ocean’s surface at night and then diving to depths of about 700 meters during daylight hours, possibly to avoid predators.